1. Field of the Invention
This invention relates generally to a system and associated method for detecting defects in a material and, more particularly, to a system and associated method for detecting defects in a material, where the system includes a transducer for coupling a multiple mode flexible excitation signal into the material, and includes a thermal imaging camera for imaging the heat created in the material as a result of the acoustic chaos or flexible excitation.
2. Discussion of the Related Art
Maintaining the structural integrity of certain structures is very important in many fields because of safety concerns, downtime, cost, etc. Loss of structural integrity is typically caused by material defects, such as cracks, delaminations, disbonds, corrosion, inclusions, voids, etc., that may exist in the structure. For example, it is very important in the power generation industry that reliable techniques are available to examine the structural integrity of turbine, generator and associated balance of plant equipment to ensure the components and systems do not suffer failure during operation. Similarly, it is very important in the aviation industry that reliable techniques are available to examine the structural integrity of the aircraft skin and structural components of the aircraft to ensure that the aircraft does not suffer structural failure when in flight. The structural integrity of turbine blades and rotors and vehicle cylinder heads is also very important in those industries. The most common method for detection of a crack or defect is visual examination by skilled personnel. But, it is known that cracks or defects that may affect the integrity of structural components may not be readily visible without the use of special techniques to aid the examiner. Therefore, various techniques have been developed in the art for the non-invasive and non-destructive analysis of different structural components and materials in many industries.
One known technique for the non-invasive and non-destructive testing of a material for defects includes treating the material with a dye penetrant so that the dye enters any crack or defect that may be present in the material. The material is then cleaned and treated with a powder that causes the dye that remains in the crack to wick into the powder. An ultraviolet (UV) light source is used to inspect the material to observe locations in the material that fluoresce as a result of the dye. This technique has the disadvantage, however, that it is highly inspector intensive and dependent because the person inspecting for the fluorescence must be skilled. Additionally, the dye does not penetrate tightly closed cracks or cracks that are not on the surface of the material.
A second known technique for inspecting a component for defects employs an electromagnetic coil to induce eddy currents in the component. The coil is moved around on the component, and the eddy current pattern changes at a crack or other defect. The complex impedance in the coil changes as the eddy current changes, which can be observed on an oscilloscope. This technique has the drawback, however, that it is also very operator intensive, and is also extremely slow and tedious.
Another known technique for detecting defects in a component employs thermal imaging of the component to identify the defects. In other thermal imaging techniques, a heat source, such as a flash lamp or a heat gun, is used to direct a planar pulse of heat to the surface of the component. The component absorbs the heat, and emits radiation in the infrared wavelengths. Certain types of defects will cause the surface temperature to cool at a different rate around the defect than for the surface temperature of surrounding areas. A thermal or infrared imaging camera is used to image the component and detect the resulting surface temperature variations. Although this technique has been successful for detecting disbonds and corrosions, it is ordinarily not successful for detecting vertical cracks in the component, that is, those cracks that are perpendicular to the surface of the component. This is because a fatigue crack looks like a knife edge to the planar heat pulse, and therefore no, or minimal, heat reflections occur from the crack making it difficult or impossible to see in a thermal image.
Thermal imaging for detecting defects in a material has been extended to systems that employ ultrasonic excitation of the material to generate the heat. An acoustic thermal effect occurs when sound waves propagate through a solid body that contains a crack or other defect causing it to vibrate. Because the faces of the crack ordinarily do not vibrate in unison as the sound waves pass, dissipative phenomena, such as friction between the faces, will convert some of the vibrational energy to heat. By combining this heating effect with infrared imaging, a very efficient, non-destructive crack detection system can be realized. Such imaging systems are generally described in the literature as sonic IR, thermosonic, acoustic thermography, etc.
The article Rantala, J., et al. “Lock-in Thermography with Mechanical Loss Angle Heating at Ultrasonic Frequencies,” Quantitative Infrared Thermography, Eurotherm Series 50, Edizioni Ets Piza 1997, pgs. 389–393 discloses such a defect detection technique. The ultrasonic waves cause the opposing edges of the crack to rub together causing the crack to heat up. Because the undamaged part of the component is only minimally heated by the ultrasonic waves, the resulting thermal images of the component show the crack as a bright area against a dark background field.
U.S. Pat. No. 6,236,049 issued May 22, 2001 to Thomas et al. titled “Infrared Imaging of Ultrasonically Excited Subsurface Defects in Materials,” assigned to the Assignee of this application, and herein incorporated by reference, discloses a thermal imaging system for detecting cracks and other defects in a component by ultrasonic excitation. An ultrasonic transducer is coupled to the component, and ultrasonic energy from the transducer causes the defects to heat up, which is detected by a thermal camera. The ultrasonic energy is in the form of a substantially constant amplitude pulse. A control unit is employed to provide timing and control functions for the operation of the ultrasonic transducer and the camera.